In recent years collagen has been widely accepted as a commercial material for the manufacture of artificial sausage casings particularly suited for the processing of pork sausages. In the manufacture of collagen casings, a collagen source, typically hide collagen, is converted into a finely divided fibrillar form and extruded in the form of a dilute collagen slurry through a die to form a tubular casing. The extruded collagen is passed into a coagulating bath which dehydrates the collagen slurry into a solid coherent collagen casing. The coagulating bath typically contains a salt such as sodium sulfate or ammonium sulfate and a small amount of alkali, e.g., sodium hydroxide or ammonia, for neutralizing excess acid which may be present in the collagen casing. The collagen casing, then, is hardened or tanned to permit further processing of the casing and to provide it with sufficient strength for its intended use. Often, a two-step tanning process is employed with the first tanning step being effected by an aluminum tanning agent, e.g., an aluminum citrate complex, and the second tanning being effected by reaction with a dialdehyde, e.g., glutaraldehyde. After tanning, the casing is inflated with air and dried in an elongated dryer with air maintained at a temperature of about 175.degree. C. and circulated at a rate of about 2,000 feet per minute. The temperature of the casing during the drying operation is held below about 90.degree. C. and usually below about 80.degree. C.
One of the difficulties in drying collagen casing, as opposed to an artificial sausage casing made of regenerated cellulose, is that collagen is much more sensitive to heat than is regenerated cellulose. Collagen shrinks on heating and if the temperature of the collagen become too high it will form gelatin. The conversion of collagen to gelatin in the dryer, of course, has disastrous effects on the resulting casing. One particular disadvantage is that the casing is sticky in its gelatinized form and may adhere to the surfaces of the guide roller and cause the line to break in the dryer.
A second difficulty peculiar to collagen and not to cellulose is that if the casing is not maintained in an inflated condition in the dryer it seals against itself and is practically impossible to separate for subsequent inflation. On the other hand, cellulose can be easily threaded through the dryer in a flat state and subsequently inflated.